Thermoplastic resin microporous membranes have been widely used, for example, as a material for separation, selective permeation, and isolation of substances: e.g., battery separators used in lithium ion secondary batteries, nickel-hydrogen batteries, nickel-cadmium batteries, and polymer batteries; separators for electric double layer capacitors; various filters such as reverse osmosis filtration membranes, ultrafiltration membranes, and microfiltration membranes; moisture-permeable waterproof clothing; and medical materials. In particular, as lithium ion secondary battery separators, polyethylene porous membranes have been suitably used which have ion permeability due to electrolyte impregnation, are excellent in electrical insulation property, electrolyte resistance, and oxidation resistance, and have a pore-blocking effect of blocking a current at a temperature of about 120 to 150° C. in abnormal temperature rise of a battery to suppress excessive temperature rise. However, when the temperature continues to rise for some reason even after pores are blocked, the polyethylene porous membrane may be ruptured because of a decrease in viscosity of polyethylene constituting the membrane and shrinkage of the membrane. This phenomenon is not a phenomenon that occurs only when polyethylene is used, and also when any other thermoplastic resin is used, this phenomenon is unavoidable at or higher than the melting point of the resin constituting the porous membrane.
In particular, lithium ion battery separators are highly responsible for battery properties, battery productivity, and battery safety, and required to have excellent mechanical properties, heat resistance, permeability, dimensional stability, pore-blocking property (shutdown property), melt-rupture property (meltdown property), and the like. Furthermore, improved adhesion to electrode material for improved battery cycle characteristics, and improved electrolyte permeability for improved productivity are required. For these reasons, studies have hitherto been conducted to laminate various modifying porous layers to a porous membrane. As the modifying porous layer, for example, polyamide-imide resins, polyimide resins, and polyamide resins, all of which have both heat resistance and electrolyte permeability, and/or fluorine resins, which have excellent electrode adhesion, are being suitably used. In addition, water-soluble or water-dispersible binders, which allow lamination of the modifying porous layer through relatively simple water washing and drying, are also being widely used. The modifying porous layer as used herein refers to a layer containing a resin that provides or improves at least one function such as heat resistance, adhesion to electrode material, or electrolyte permeability.
Furthermore, it is necessary to increase the area that can be loaded into a container in order to increase the battery capacity, and it is expected that separators as well as electrode sheets will become thinner and thinner. As a porous membrane becomes thinner, however, the porous membrane tends to be deformed in the planar direction; consequently, a battery separator in which a modifying porous layer is laminated to the thin porous membrane can undergo peeling-off of the modifying porous layer during processing, slitting, or battery assembling, and it becomes more difficult to secure the safety.
Furthermore, it is expected that the speed of battery assembling will be faster to achieve a cost reduction, and high adhesion between the porous membrane and the modifying porous layer to withstand high-speed processing are required, by which troubles such as peeling-off of the modifying porous layer are less likely to occur even in such high-speed processing. However, when a resin contained in the modifying porous layer is sufficiently infiltrated into a polyolefin porous membrane serving as a substrate in order to improve the adhesion, the amount of increase in air resistance disadvantageously increases.